1. Field of the Invention
The present invention relates to catalysts capable of effectively purifying hydrocarbon (HC) included in exhaust gases of which temperatures range from a low temperature region to a high temperature region.
2. Description of Related Art
Conventionally, oxidation catalysts which carry noble metals such as Pt (platinum) in porous carriers composed of alumina or the like have been used as catalysts for purifying exhaust gases of motor vehicles. With these oxidation catalysts, HC and CO (carbon monoxide) in exhaust gases are oxidized and purified.
In addition, by adjusting the air-fuel ratio to a stoichiometric air-fuel ratio, catalysts which carry noble metals such as Pt in porous carriers composed of alumina or the like operate to oxidize CO and HC in exhaust gases and reduce NOx (nitrogen oxides) therein at the same time, and accordingly have been called three-way catalysts. As such three-way catalysts, there have been widely used catalysts, each having a porous carrier layer composed of xcex3-alumina, which is formed on a heat-resistant honeycomb substrate composed of cordierite or the like, and by which noble metals such as Pt, Rh (rhodium) or the like are carried, for example.
These oxidation catalysts and three-way catalysts, however, have problems that the catalyst reaction thereof does not occur until the noble metals carried thereby exceed their activity temperatures. Accordingly, where engines are started or are in cold states, the temperatures of exhaust gases are low, and the noble metals do not reach their activity temperatures, whereby HC and NOx are difficult to be purified.
Where engines are in cold states, an air-fuel mixture of which the fuel concentration is higher than that where engines are normally operated, is supplied, and accordingly, the amount of HC included in exhaust gases is large. In addition, noble metals are not activated at temperatures lower than about 300xc2x0 C. Therefore, at low temperatures such as the temperatures when engines are started, it is impossible to purify HC included in exhaust gases sufficiently. Accordingly, it has been particularly desired to purify HC effectively where the engines are in cold states or are started.
Zeolite which has been also referred to as molecular sieves has micro pores of which the diameter is less than 2 nm, similarly to dimensions of molecules, and accordingly has been used as adsorbents, and catalysts in many reactions. Recently, the application of zeolite having the above-described properties to catalysts for purifying exhaust gases of motor vehicles has been investigated. Zeolite itself is, however, low in carrying properties of noble metals, and if carrying, the amount of carried noble metals is too small to exhibit sufficient oxidizing ability.
For this reason, recently, zeolite has been used as adsorbent for HC. As shown in publications of unexamined Japanese Patent applications Nos. Hei 5-057148 and Hei 6-154538, there have been developed exhaust gas purifying devices wherein adsorbent such as zeolite is disposed upstream of an oxidation catalyst or three-way catalyst in an exhaust gas flowing direction. With these exhaust gas purifying devices, when engines are at low temperatures upon starting the engines, for example, HC in exhaust gases is first adsorbed on the adsorbent. When the temperature of exhaust gas is elevated, the adsorbed HC is desorbed, and the desorbed HC is oxidized and purified with the oxidation catalyst or three-way catalyst which is arranged downstream of the adsorbent and of which the temperature is elevated to its activity temperature or more.
And in publications of unexamined Japanese Patent applications Nos. Hei 11-210451, Hei 11-104462, for example, there are proposed integral honeycomb monolithic catalysts for purifying exhaust gases, each having a coating layer composed of powder of adsorbent for HC as a lower layer and another coating layer composed of powder of oxidation catalyst or three-way catalyst as an upper layer.
With these catalysts for purifying exhaust gases, at a low temperature, HC in exhaust gases passes the upper layer which does not reach its activity temperature, and is adsorbed on the adsorbent for HC in the lower layer. As the temperature of exhaust gases is elevated, HC which has been adsorbed is desorbed, and is oxidized and purified with the oxidation catalyst or three-way catalyst in the upper layer, of which the temperature increases to its activity temperature or more.
With these exhaust gas purifying devices or catalysts, when engines are in cold states or are started, HC included in exhaust gases is adsorbed on the adsorbent for HC to restrain the emission thereof. And, when the temperatures are elevated, HC desorbed from the adsorbent for HC and included in exhaust gases is oxidized and purified with the oxidation catalyst or three-way catalyst. Accordingly, the emission of HC can be restrained from the lower temperature region to the higher temperature region. The amount of HC which is emitted unburnt can be reduced.
However, in the devices for purifying exhaust gases wherein the adsorbent such as zeolite is arranged upstream of the oxidation catalyst or three-way catalyst, heat of exhaust gases is absorbed by the upstream adsorbent to block the elevation of the temperature of the downstream oxidation catalyst or three-way catalyst so that it takes a long time to elevate the temperatures of noble metals carried by the catalyst up to the activity temperatures thereof, and the HC-purification rate does not increase until the temperatures of the noble metals are elevated up to the activity temperatures thereof.
For solving the above-described problem, it is more profitable to use catalysts, each having a layer of an adsorbent for HC as its lower layer and an oxidation catalyst layer or three-way catalyst layer as its upper layer. However, considering the recent tightened emission regulation, much improvement of the purification performance is desired.
The present invention has been made considering these circumstances, and has an object of much improving the HC-purification performance by providing zeolite as an adsorbent for HC in a most suitable arrangement.
The catalyst for purifying exhaust gases in accordance with the present invention includes a substrate having heat resistance, a coating layer composed of a carrier containing alumina as a main ingredient, and zeolite, and formed on a surface of the substrate, and noble metals which are carried by the coating layer. The composition ratio of alumina and zeolite in the coating layer ranges from 5:1 to 1:1 by weight.
It is preferable that a layer of an adsorbent for HC (hereinafter referred to as HC-adsorbent) is further formed between the substrate and the coating layer.
Another catalyst for purifying exhaust gases in accordance with the present invention includes a substrate having beat resistance, an HC-adsorbent layer formed on a surface of the substrate a lower catalyst layer composed of a porous carrier which carries Pd, and formed on a surface of the HC-adsorbent layer, and an upper catalyst layer composed of a porous carrier which carries Pt and Rh, and formed on a surface of the lower catalyst layer.
It is preferable that at least one of the lower catalyst layer and upper catalyst layer in the above-described catalyst for purifying exhaust gases contains at least oxide containing Ce.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which from a part of this specification.